USING ELECTRICAL RESISTIVITY TO DETERMINE THE IMPACT OF URBANIZATION ON A BEDROCK STREAM SYSTEM

The hyporheic exchange cycles nutrients, moderates stream temperature and improves water quality. Investigating this exchange in fractured bedrock streams set in urban areas can be difficult because of the complex flow pathways. Understanding this process can better guide future projects to minimize the impact to the environment. This research uses timelapse electrical geophysical methods to investigate the hyporheic exchange in an urban impacted bedrock stream system. The research site is located along the headwater reach of Cooper Creek in Sycamore Township, Ohio. Since 1948, a large portion of the contributing area has been converted to a public park with baseball diamonds and drainage tiles replacing natural ephemeral tributaries; a sanitary sewer line was installed along the streambank; and the reach has been beheaded by development. Changes to the stream system can be observed since urbanization and evidence shows the stream has undergone morphology and stream flow changes. At the site, rebar stakes were installed along 8 profile lines to create permanent electrode arrays. The arrays are installed in three different zones. Zones 1 and 2 are installed in an area where the sanitary sewer line runs along the stream bank. Zone 1 experiences consistent flow while Zone 2 is located along the ‘dry reach’, an area of the stream that experiences intermittent flow. Zone 3 is located furthest upstream in the area where the flow of the stream water disappears. These areas were chosen to gather insight on the hyporheic exchange near the sewer line and to help investigate the cause of the disappearing stream. The arrays were installed in December 2022 with timelapse data collection beginning January 2023. A Dipole-Dipole and a Wenner array are completed for each profile line. There has been a total of 30 data collection days so far. Timelapse inversions were performed using the open-source Resipy software to quantify changes in electrical resistivity profiles through time. Decreases in resistivity within the data collected thus far suggest the utility line is having an impact on the flow of groundwater in the immediate vicinity around the utility. Fracture zones and variable bedrock topography are also evident in the data collected so far.